The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and, more particularly, to an image forming apparatus of the type including an intermediate transfer device for transferring a toner image from a photoconductive element or similar image carrier to an intermediate transfer body by an electric field formed in a portion where the image carrier and transfer body contact each other.
An electrophotographic copier, printer or similar image forming apparatus capable of producing a color image by superposing toner images of different colors with an image transfer device is conventional. Generally, the image transfer device uses either one of a transfer drum system and an intermediate transfer body system. In the transfer drum system, a paper or similar recording medium is wrapped around a film fitted on a transfer drum, and toner images of different colors are sequentially transferred from a photoconductive element or image carrier to the recording medium one above the other. Because the recording medium must be electrostatically retained on the film, the film is formed of an insulator.
The intermediate transfer body system is such that toner images of different colors are directly transferred from a photoconductive element to an intermediate transfer body one above the other and then collectively transferred from the intermediate transfer body to a recording medium. Because the recording medium does not have to be wrapped around the intermediate transfer body, the transfer body can be formed of a material having a medium resistance (volume resistivity of 10.sup.7 .OMEGA.cm to 10.sup.14 .OMEGA.cm) as distinguished from an insulator. A charge deposited on a material having a medium resistance naturally attenuates with a preselected time constant. The image transfer device with such an intermediate transfer body is therefore advantageous over the transfer drum type device in that it does not need discharging means for forcibly dissipating a transfer charge. This kind of image transfer device reduces ozone and saves power while achieving paper-free and full-page copy features.
However, the problem with the intermediate transfer body formed of a medium resistance material is that the resistance of such a material is irregular and apt to vary due to the varying environment and aging, rendering the transfer body electrically unstable. As a result, characters and lines transferred to the intermediate transfer body are sometimes blurred or otherwise defective due to the scattering of toner.
In light of the above, Japanese Patent Application Nos. 8-283210 and 9-150197 propose an image transferring method and an image forming apparatus capable of dissipating a transfer charge at a portion where the image carrier and intermediate transfer body contact each other (transfer nip). With such a method and apparatus, it is possible to effectively reduce the above defective images even when the intermediate transfer body is formed of a medium resistance material. However, a series of researches and experiments showed that a desired transfer efficiency was not achieved or transferred images were deteriorated, depending on transfer charge applying conditions and discharging conditions.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-272222, 10-48970 and 10-73999.